1. Field of the Invention
The present invention relates to a disk drive with a unique servo pattern of special calibration bursts that are "disposably" written in a data region, used for independently calibrating a read head and, if desired, later disposed of by being written over with data.
2. Description of the Prior Art and Related Information
A conventional disk drive contains a disk with a plurality of concentric data tracks and a "head" which generally comprises a "slider" that carries a read transducer and a write transducer. The drive has "servo" information recorded on this disk or on another disk to determine the position of the head. The most popular form of servo is called "embedded servo" wherein the servo information is written on the disk in a plurality of servo sectors or "wedges" that are interspersed between data regions. Data is conventionally written in the data regions in a plurality of discrete data sectors. Each data region is preceded by a servo wedge. Each servo wedge generally comprises a servo header (HDR) containing a track identification (TKID) field and a wedge number (W#) field, followed by at least two angularly successive servo burst regions that define a plurality of burst pair centerlines. Each servo burst is conventionally formed from a series of magnetic transitions defined by an alternating pattern of magnetic domains. The servo control system samples the servo bursts with the read transducer to align the transducer with or relative to a burst pair centerline and, therefore, with or relative to a particular data track.
The servo control system moves the transducer toward a desired track during a "seek" mode using the TKID field as a control input. Once the transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the transducer head over that track in a "track follow" mode. The transducer reads the servo bursts to produce a position error signal (PES). The PES has a particular value when the transducer is at a particular radial position relative to a burst pair centerline defined by the bursts and, therefore, relative to the data track center. The desired track following position may or may not be at the burst pair centerline or data track center.
The width of the write transducer is desirably narrower than the data track pitch. The servo information is recorded, therefore, to define a data track pitch that is slightly wider than the write transducer to provide room for tracking error. The servo information is usually recorded to define a data track pitch that is about 25% wider than the nominal width of the write transducer and conversely, therefore, the nominal write transducer is about 85% of the track pitch. The percentage is not exactly 85% for every transducer, however, since the width of the write transducer will vary from nominal due to typical manufacturing distributions.
FIGS. 1A and 3A show disks 12 having a plurality of servo wedges 211 comprising servo sectors 511 disposed in concentric tracks across the disk, and corresponding data regions 212 comprising data sectors 512 disposed in the concentric tracks between the servo wedges 211. For clarity, the disks 12 are simplified to show only four wedge pairs 211, 212 whereas a typical disk is divided into 70-90 wedge pairs. The servo wedges 211, moreover, are greatly exaggerated in width relative to the width of the data regions 212. Finally, the disks 12 of FIGS. 1A and 3A have only one annular "zone" of data sectors 512 from the inner diameter (ID) of the disk 12 to the outer diameter (OD), whereas an actual disk 12 usually has multiple concentric zones in order to increase the data capacity of the drive by packing more data sectors 512 in the larger circumference tracks near the OD.
FIGS. 1A and 1B, in particular, schematically represent an older disk drive having a disk 12 wherein each servo wedge 211 only has two angularly successive servo burst regions 221, 222 that contain A and B bursts, respectively. The servo bursts A,B moreover, are 100% bursts that stretch radially from data track center to data track center such that there is only one burst pair centerline (e.g. 412) per data track (an annular collection of data sectors 512). As shown in FIG. 1B, for example, the A and B bursts A(12), B(12) define a burst pair centerline 412 that coincides with the data track centerline (not separately numbered).
The disk drive of FIGS. 1A and 1B has a head slider 200 in which the same inductive transducer 201 both reads and writes data. When the R/W transducer 201 is on track center in such arrangement, it detects equal signal amplitudes from the two angularly successive, radially offset A and B bursts such that A-B=0. If the R/W transducer 201 is offset one way or the other from the burst pair centerline 412, an inequality exists between the signal amplitudes, such that A-B.noteq.0. The inequality is usually expressed as an algebraic position error signal (PES) such as A-B discussed above. The PES should be proportional to the position error (PE), i.e. to the mechanical offset of the R/W transducer 201 relative to the burst pair centerline 412.
The servo pattern of FIGS. 1A and 1B. however, contains nonlinear regions because the R/W transducer 201 becomes saturated when it is in certain positions. In particular, with only two 100% bursts and one burst pair centerline per data track, if the transducer is displaced too far from the burst pair centerline, the R/W transducer 201 may be completely over one of the bursts and no longer pass over any part of the other burst. The maximum linear signal (PES) occurs when displacement is one half of the transducer's physical width. As shown in FIG. 2, for example, the 80% R/W transducer 201 of FIG. 1B can only be displaced by a maximum of 40% of a data track pitch from the burst pair centerline 412 and still pass over at least a portion of both bursts A(12), B(12) to develop a linearly varying A-B position error signal. The nonlinear regions where the head position is ambiguous are called "blind spots" or "gaps" 413, 414.
FIGS. 3A and 3B schematically represent the industry's solution to the gaps 413, 414 of FIG. 2. Here, the disk drive includes two additional, angularly successive servo burst regions 231, 232 to provide C and D bursts that fill the gaps between the A and B bursts. The C and D bursts are placed in "quadrature" with the A and B bursts in that the edges of the C and D bursts are aligned with the centers of the A and B bursts. With four 100% bursts A, B, C, D positioned in quadrature, there are two burst pair centerlines (e.g. 411, 412) per data track pitch, i.e. one burst pair centerline every 50% of a data track pitch. The 80% R/W transducer 201, therefore, will always pass over both parts of an A/B pair or a C/D pair because it is always within 25% of a data track pitch from an A/B or C/D burst pair centerline. FIG. 4 shows how the linear portions of the C-D position error signal which surround the C/D burst pair centerlines are radially aligned with the gaps in the A-B position error signal, and vice versa.
The industry recently began using magnetoresistive heads (MR heads) which contain two separate transducers--an inductive transducer for writing and a magnetoresistive transducer for reading. FIG. 5 is a schematic plan view of a typical MR head 100 having a slider 110 which carries an inductive write transducer 101 and a separate, magnetoresistive read transducer 102.
An MR head 100 is advantageous in providing an improved signal-to-noise-ratio (SNR) to recover data in disk drives of high areal density. However an MR head also presents a number of disadvantages. In particular, the separate read and write transducers 102, 101 are necessarily spaced apart from one another along the length of the slider 110. As a result, their radial separation varies from ID to OD as the MR head 100 is moved in an arc by a swing-type actuator.
The drive industry presently compensates for the variable radial separation between the transducers 101, 102 by "micro-jogging" the read transducer 102 relative to a given burst pair centerline by an amount corresponding to the radial displacement at that cylinder. This jogging solution generally requires separate and distinct track following procedures for reading and writing. The drive can micro-jog when writing data or when reading data, but it is preferable to track follow on the burst pair centerline while writing and only jog when reading so that the data is consistently written to the same location. In a typical MR head drive, therefore, the read transducer 102 track follows a burst pair centerline at the "null" position where the average PES=0 and the write transducer 101 records the data track offset toward the ID or the OD by the amount of radial separation between the read and write transducers 102, 101 at this cylinder. For reading, the read transducer 102 is "micro-jogged" away from the null position of the burst pair centerline where the average PES=0, in order to align the read transducer 102 with the recorded data.
An MR head 100 is sometimes called a "Write Wide/Read Narrow" head because the inductive write transducer 101 is usually wider than the magnetoresistive read transducer 102, as shown in FIG. 5. The relatively narrow read transducer 102 causes problems with micro-jogging because the maximum displacement which provides reasonably linear PES is one half of the read transducer's physical width. In particular, where the write transducer 101 is about 80% of a data track pitch, the typical read transducer 102 is only about 66% of a data track pitch and micro-jogging is limited to half that amount, i.e. to about .+-.33%. As described in more detail below with reference to FIG. 7, however, the actual linear micro-jogging maximum range is even less than 33%, because the magnetoresistive read transducer 102 also suffers from an uneven "microtrack profile" (i.e. is not uniformly sensitive across its width). The transducer 102 is additionally subject to "side reading" (i.e. is sensitive to nearby transitions not actually under the transducer). Because of the narrow width and nonlinear response, and unless otherwise corrected, the typical 66% magnetoresistive read transducer 102 can only be micro-jogged by about .+-.20% of a data track pitch and still provide a servo signals that reasonably vary in linear proportion to displacement from a burst pair centerline.
The drive industry conventionally reduces the problems of narrow width and nonlinear response by adding more, closely spaced, burst pair centerlines. The additional burst pair centerlines are added by packing more servo bursts into the circumferential or radial dimensions of the disk. Adding more servo bursts in the radial dimension is generally preferred because it does not increase the angular width of the servo wedges and thereby reduce the area available for storing data. Adding more servo bursts in the radial dimension does, however, require bursts that are narrower than 100% of a data track pitch. For example, using four 2/3 track pitch bursts A, B, C, D on 1/3 track pitch offsets to create a 1/3, 1/3, 1/3 pattern of three burst pair centerlines per data track pitch ensures that the read head is always within 1/6th of a data track pitch (16.67%) from a burst pair centerline, i.e. well within the nonsaturated range of about .+-.20% for a typical magnetoresistive read transducer.
As shown in FIG. 8, however, the farther the magnetoresistive read transducer 102 is displaced from a burst pair centerline, the more the corresponding servo signals or PES vary from the ideal. This increasingly nonlinear variance presents a problem even with the limited 16.67% micro-jogging capability required in the context of a 1/3, 1/3, 1/3 servo pattern.
Moreover, manufacturing a disk drive with a 1/3, 1/3, 1/3 pattern is relatively expensive because of the time needed to record the additional servo bursts as indicated below. If possible, therefore, it is desirable to calibrate the magnetoresistive read transducer 102 to increase its reasonably linear range beyond .+-.16.67% to at least .+-.25% so that the transducer 102 may be effectively micro-jogged .+-.25% using a less expensive, 1/2, 1/2 pattern that requires only two burst pair centerlines per data track.
A manufacturing fixture called a servo track writer (STW) is ordinarily used only to record servo information on the disks of a Head Disk Assembly (HDA) by mechanically moving the HDA's actuator to a given reference position that is precisely measured by a laser interferometer or other precision measurement device. The HDA is then driven to record the servo bursts and other servo track information for that position. The process of precisely measured displacement and servo track writing is repeated to write all required servo tracks across the disk.
We could theoretically use the STW to calibrate the magnetoresistive read transducer 102 by leaving the drive in the STW after recording the servo information. The calibration is possible because the STW provides us with the actual displacement from a reference position (i.e. a burst pair centerline where the PES=0). In general operation, the STW would move the HDA's actuator to a plurality of known off-center positions, and the HDA would read the servo signals at those positions and then associate the servo signals or resultant PES with actual displacement to develop a compensation table or formula.
Using the STW to perform the actual calibration, however, is undesirable for several reasons. First, an STW is a very expensive piece of machinery, costing $100,000.00 or more and therefore available in limited quantities. Increasing the time each HDA spends in the STW, therefore, adversely impacts production time and cost. Second, the STW undesirably consumes floor space. Finally, it is undesirable to calibrate the drive in the STW because the calibration must be performed prior to and independent of the detailed self calibration process which the disk drive performs later in the manufacturing cycle. This is a significant disadvantage because the parameters of the servo channel may change due to adjustments in DC bias current applied to the MR transducer or other factors. Accordingly, the calibrations made with the STW may become inaccurate or entirely invalid.
There remains a need, therefore, for a disk drive that can independently calibrate its read transducers after leaving the STW and without significantly reducing the disk drive's data storage capacity.